1. Field of the Invention
The present invention relates to a magnetic video signal reproducing apparatus which can magnetically record and reproduce video signals.
2. Description of the Prior Art
An example of prior art magnetic video signal reproducing apparatus will be described hereinbelow with reference to FIGS. 1A and 1B to FIG. 6.
In the prior art video signal reproducing apparatus (e.g. video tape recorder) as shown in FIG. 1A, a magnetic tape TT is obliquely wound around about a half of the circumference of a rotating drum X and further moved in the travel direction (the arrow direction) TT1. In addition, the rotating drum X is rotated in the clockwise direction (the arrow direction) X1, in order to reproduce video signals with the selective use of two first and second magnetic heads H1 and H2 provided with two different azimuth angles with respect to each other and arranged so as to be opposed at an angle of 180 degrees on the rotating drum X.
In the case where the magnetic tape TT is moved for high speed video signal reproduction at a speed higher than the ordinary magnetic tape travel speed, video signals are reproduced with the use of other two third and fourth magnetic heads H3 and H4 also provided with two different azimuth angles with respect to each other and arranged so as to be opposed at an angle of 180 degrees on the rotating drum X. Here, the third and fourth magnetic heads H3 and H4 are arranged away from the first and second magnetic heads H1 and H2 by an angle corresponding to approximately one horizontal line period (referred to as "1H" hereinafter), respectively. The azimuth angles of the first and fourth magnetic heads H1 and H4 are equal to each other, and the azimuth angles of the second and third magnetic heads are equal to each other, respectively.
In the high speed reproduction operation, since the one magnetic head passes through tracks written at different azimuth angles, the amplitude of the reproduced signals changes large and small alternately. For instance, in FIG. 1B, the signal reproduced by the first magnetic head H1 is of waveform as shown by H1a and the signal reproduced by the third magnetic head H3 is of waveform as shown by H3a.
An example of the prior art magnetic reproducing apparatus for reproducing the video signals as described above will be described hereinbelow with reference to FIG. 2. The apparatus is composed of a switch circuit (SW) 1; an FM (frequency modulation) level detecting circuit 2; a low-pass filter (LPF) 3; a high-pass filter (HPF) 4; a chrominance signal reproducing circuit 5; a first time base correcting circuit (TBC1) 6; a luminance signal reproducing circuit 7; a second time base correcting circuit (TBC2) 8; a synchronizing separator circuit (SS) 9; a latch circuit 10; and an adder circuit 11.
The signals H1a and H3a reproduced by the first and third magnetic heads H1 and H3 are amplified through two preamplifiers (not shown) to a predetermined amplitude, respectively and then applied to the switch circuit 1 and the FM level detecting circuit 2. The amplitudes of the two reproduced signals H1a and H3a are detected and compared with each other by the FM level detecting circuit 2 for binarization. The binarized output signal of the FM level detecting circuit 2 is supplied to the latch circuit 10.
The operation of the latch circuit 10 will be described with reference to FIG. 3. First, a horizontal synchronizing signal 9a as shown in FIG. 3(b) is separated synchronously from the reproduced luminance signal 7a as shown in FIG. 3(a) through the synchronizing separator circuit 9. The separated horizontal synchronizing signal 9a is applied to the latch circuit 10 as a clock signal. In response to the rising edge of this clock signal 9a, the latch circuit 10 latches the output signal 2a of the FM level detecting circuit 2 as shown in FIG. 3(c), and outputs a latch circuit output signal 10a as shown in FIG. 3(d) to the switch circuit 1.
In response to the latch circuit output signal 10a, the switch circuit 1 shown in FIG. 2 outputs a switch circuit output signal 1a by switching the two input signals H1a and H3a, respectively. That is, the switch circuit output signal 1a is the signal H1a reproduced through the first magnetic head H1 where the output signal 10a is at a high level, and the signal H3a reproduced through the third magnetic head H3 where the output signal 10a is at a low level, for instance.
As described above, since the horizontal synchronizing signal 9a is used as the clock signal to the latch circuit 10, it is possible to set the switch timing of the reproduced signals within the horizontal blanking period. Therefore, it is possible to generate the noise due to the discontinuity in the FM modulation signal caused when the reproduced signals H1a and H3a are switched, only within the horizontal blanking period (during the period free from the visual problems).
Further, the output signal la is applied to the low-pass filter 3 for separating the chrominance signal converted into the low frequency range on the basis of frequency (the low frequency-converted chrominance signal) and to the high-pass filter 4 for separating the modulated luminance signal also on the basis of frequency. The low frequency-converted chrominance signal 3a obtained through the low-pass filter 3 is supplied to the chrominance signal reproducing circuit 5, and the signal 4a obtained through the high-pass filter 4 is supplied to the luminance signal reproducing circuit 7. The chrominance signal reproducing circuit 5 restores the phase rotation of 90 degrees per 1H (one horizontal line period) implemented when the input signal is converted into a low frequency range in the recording operation thereof, on the basis of the synchronizing signal 9a and further converts the restored chrominance signal into the high frequency range in order to obtain the reproduced chrominance signal 5a. The obtained chrominance signal 5a is supplied to the first time base correcting circuit 6. On the other hand, the luminance signal reproducing circuit 7 supplies the reproduced luminance signal 7a obtained by demodulating the input signal in frequency to both the synchronizing separator circuit 9 and the second time base correcting circuit 8.
The first and second time base correcting circuits 6 and 8 correct the time bases of the reproduced chrominance signal 5a and the reproduced luminance signal 7a, respectively on the basis of the horizontal synchronizing signal 9a. The respective chrominance signal 6a and luminance signal 8a thus obtained are outputted to transmission paths (not shown) and the adder circuit 11. Further, the video signal 11a obtained by adding both the signals 6a and 8a is outputted to another transmission path (not shown).
As described above, in the high speed reproduction operation, since the magnetic heads are switched during the horizontal blanking period and further the input signal is demodulated in frequency, the switching noise superposed upon the outputted luminance signal 7a can be generated only the horizontal blanking period so as not to be noticeable on the picture display.
In the prior art video signal reproducing apparatus as described above, however, since the signal reproduced by the first magnetic head H1 is not in phase with the signal reproduced by the third magnetic head H3 at the timing when the magnetic heads are switched, when the horizontal frequency of the reproduced luminance signal 7a changes abruptly, a skew (asymmetrical portion) is inevitably produced. Consequently, even if the phase difference is corrected by the second time base correcting circuit 8, there still exists a problem in that the outputted luminance signal 8a includes a skew in the horizontal line immediately after the magnetic heads have been switched. In the reproduced chrominance signal 5a, on the other hand, since the order of the above-mentioned rotation correction is discontinuous, it takes some time to pull in the phase by an APC (automatic phase control) circuit (not shown), thus causing another problem in that the color is disturbed in the horizontal line immediately after the magnetic heads have been switched.
The problem in the outputted luminance signal 8a will be described in further detail hereinbelow with reference to FIG. 4. FIG. 4(a) shows the reproduced luminance signal 7a-1 related to the first magnetic head H1 obtained when the magnetic heads are not switched; and FIG. 4(b) shows the reproduced luminance signal 7a-2 related to the third magnetic head H3 obtained when the magnetic heads are not switched. In the phase relationship as shown by FIG. 4(a) and FIG. 4(b), the assumption is made that the magnetic heads are switched. In the case where the first magnetic head H1 is switched to the third magnetic head H3 at the switch timing "A", the reproduced luminance signal 7a-3 as shown in FIG. 4(c) can be obtained. Further, in the case where the third magnetic head H3 is switched to the first magnetic head H1 at the switch timing "B", the reproduced luminance signal 7a-4 as shown in FIG. 4(d) can be obtained. The above-mentioned luminance signals 7a-3 and 7a-4 thus obtained have a skew, respectively.
The operation of the second time base correcting circuit 8 for correcting the time base of these signals will be described with reference to FIG. 4(e). The reproduced luminance signal 7a-5 as shown in FIG. 4(e) are written in order in first to seventh line memories M1 to M7 of the second time base correcting circuit 8, for a predetermined time period after the falling edge of the horizontal synchronizing signal 9a, in response to the clock signal (the horizontal synchronizing signal 9a) following the jitter of the reproduced luminance signal 7a-5. Further, the written reproduced luminance signal 7a is read, as the outputted luminance signal 8a as shown in FIG. 4(f), in response to the clock signal of a stable frequency. Here, since the outputted luminance signal 8a related to the fourth line memory M4 includes two horizontal synchronizing signals, a skew is inevitably generated. Further, since the outputted luminance signal 8a related to the sixth line memory M6 is not perfect in waveform shape (at the front half portion), thus generating another skew in the same way.
The problem in the outputted chrominance signal 6a will be described with reference to FIG. 5. FIG. 5(a) shows the output signal 10a of the latch circuit 10; and FIG. 5(b) shows the rotation of the reproduced low frequency-converted chrominance signal 3a obtained when the magnetic heads are switched under the ideal conditions. As shown in FIG. 5, when the output signal 10a of the latch circuit 10 is at a low level, the phase is shifted in such a way as to lag behind in sequence. On the other hand, when the output signal 10a is at a high level, the phase is shifted in such a way as to lead in sequence. As a result, it is possible to maintain the continuity of the phase.
In the prior art apparatus, however, the phase continuity is not necessarily maintained always in practice. The phase discontinuity sometimes occurs for instance in a low frequency-converted chrominance signal 3a-1 at the timings P1 and P2 as shown in FIG. 5(c).
The problem caused by the phase discontinuity as described above will be described with reference to FIG. 6, in which a prior art color rotation processor BB is shown. This processor is composed of a 4-phase signal generating circuit 63, a switching circuit 64, a frequency converting circuit 65, a 2-bit counter circuit 66, a first phase comparing circuit (P/D1) 90, a second phase comparing circuit (P/D2) 91, and a quartz oscillating circuit 92.
The frequency converting circuit 65 multiplies the output signal 64a of the switching circuit 64 by the reproduced low frequency-converted signal 3a (obtained through the low-pass filter 3 shown in FIG. 2) for conversion into a high frequency range, and outputs a high frequency-converted signal bb (3.58 MHz). The outputted signal bb is supplied to the second phase comparing circuit 91 for comparison in phase with the signal from the quartz oscillating circuit 92 (a stable oscillation source). The second phase comparing circuit 91 feedbacks the compared results to a voltage controlled oscillating circuit (not shown) for generating a local signal (4.2 MHz) included in the 4-phase signal generating circuit 63. In summary, the circuit shown in FIG. 6 forms an APC (Automatic Phase Control) loop for obtaining a stable high frequency-converted signal bb.
In addition, the high frequency-converted signal bb is also applied to the first phase comparing circuit 90 for comparison in phase with the signal supplied from the quartz oscillating circuit 92. In case where there exists a discontinuity in phase of the above-mentioned color rotation, the first phase comparing circuit 90 supplies a control signal to the 2-bit counter circuit 66 to restore the phase compulsorily. In other words, the 2-bit counter circuit 66 applies signals 66a, based on the horizontal synchronizing signal 9a, the latch circuit output signal 10a and the control signal, to the switching circuit 64, so that the switching circuit 64 can selects any one of the first to fourth local oscillation signals S0 to S3. As described above, in the prior art apparatus, the discontinuity in phase of color rotation has been corrected in accordance with the feedback loop thus constituted as described above.
In the prior art configuration, however, since the phase discontinuity can be detected only after the high frequency-converted signal bb has been outputted, the high frequency-converted signal bb is not perfectly continuous and therefore the phase discontinuity still remains, thus raising a problem in that color is disturbed in the horizontal line immediately after the magnetic heads have been switched.